1. Field of the Invention
The present invention relates to cable constructions and, more particularly, is directed towards a cable assembly having at least one shielded conductor, as well as to a method of making and using same in combination with a standard, telephone-type modular plug.
2. Description of the Prior Art
A wide variety of cable assemblies that have one or more shielded conductors are known. Such cable assemblies are utilized in various applications where it is necessary to shield a low level, information-bearing electrical signal from spurious external electrical interference. It is well known that such shielding may be accomplished by surrounding the information carrying conductor with a metal shield that, in one fashion or another, is externally grounded. Such a grounded shield effectively prevents the signal on the wire from being distorted by externally generated electrical noise or other interference.
Miniature modular plugs and mating jacks have recently gained wide popularity, especially in the telephone industry. Such miniature plugs, as examplified by U.S. Pat. No. 4,002,392 to Hardesty, are characterized by their ability to rapidly terminate a multi-conductor cable. A typical multi-conductor cable utilized with such plugs comprises a substantially planar array of conductors which are individually insulated and are then encapsulated in an outer jacket to maintain precise physical positioning thereof. A portion of the outer jacket is removed to expose the ends of the insulated conductors prior to insertion into a modular plug, and subsequent termination. Termination is achieved by individually piercing each of the insulated conductors with a small, flat insulation-piercing conductive terminal which becomes locked in place in the plug. The plug also includes means for securely gripping and thereby retaining the jacketed portion of the cable, and may also provide strain-relief means for the unjacketed terminated insulated conductors.
One advantage of the modular plugs described above is that, after termination of a multi-conductor cable therein, the plug may be rapidly connected and disconnected to a mating jack, as is well known in the art. An integral locking tab is provided on the plug for maintaining same securely within the jack, and for readily releasing the plug from the jack when desired.
While widely utilized in the telephone industry, such miniature plugs and jacks have a small but steadily increasing market in other applications, such as those which simply require a low signal level (e.g. 12 volts) interconnect cable between two pieces of electrical equipment. The low level signals on such cables frequently, however, must be transmitted in an environment where shielding of one or more of the insulated conductors is desirable, or even critical, to ensure preservation of the information content of the signals transmitted in the conductors. However, despite the growing need, a practical and inexpensive multi-conductor cable, having one or more shielded insulated conductors, which may be utilized with the popular miniature, modular telephone-style plugs, has yet to be developed.
Several different types of shielded cables are commonly known, but each suffers from one or more disadvantages as respects their cost, ease of termination, cable flexibility, or quick-disconnect ability. One common construction employs a plurality of metal strands which are either braided or wrapped in a spiral fashion about one or more insulated conductors. One disadvantage of such a construction is that the extra thickness of the metal strands makes the resulting shielded conductor considerably larger than an equivalent unshielded conductor. Thus, if both a shielded and unshielded conductor are jacketed in a single cable, the unequally sized conductors result in an unbalanced construction which is difficult to jacket smoothly and uniformly, and may therefore require specialized equipment for manufacture. Further, connection to the shield is slow and therefore quite costly from a labor standpoint. For a braided shield, for example, the individual braids must be manually unwoven, and then manipulated to one side of the cable and terminated, usually by soldering. For a wrapped shield, the wrapped strands must be unfurled from the insulated conductor, and then twisted together for termination. These types of constructions simply do not lend themselves to be rapidly terminated, especially in a miniature, modular, telephone-style plug which is designed to receive precisely aligned conductors of a predetermined size, and which also terminate the conductors by piercing the insulation, rather than by soldering. The oversized braided or wrapped shielded conductors described above simply do not fit into such modular plugs.
Another common type of shielded cable utilizes a semiconductive plastic material applied over one or more insulated conductors. The semi-conductive material is generally extruded around the insulation of the conductor desired to be shielded. The thinness wall thickness, however, that can be extruded is approximately 0.004 inch. While relatively thin when compared with the metal stranded shielded conductors described above, such a thickness nevertheless also results in an unbalanced construction and oversized conductors, thereby admitting of some of the same problems described above with respect to the metal strand shields. Further, connection techniques for such plastic shields have not been fully perfected and leave much to be desired from the standpoints of quality and reliability. These oversized shielded conductors also do not fit within the standard telephone-type modular plugs.
A third type of shielded cable assembly is exemplified by the construction described in U.S. Pat. No. 3,775,552 to Schumacher. Such a construction utilizes a metal foil and polymer laminate which surrounds both the insulated conductor and a drain wire which is externally grounded. The drain wire contacts the foil-polymer laminate to provide the desired shielding of the insulated conductor positioned therewithin. The presence of the drain wire causes the shielded conductor to be eccentric, making it larger than a corresponding unshielded conductor, which results in an unbalanced construction as described hereinabove for the other prior art assemblies. Further, the foil shield cannot be terminated without the metal drain wire. To attempt to utilize such a construction in a miniature, telephone-style modular plug would require the foil shield to be unfurled and then cut off along with the drain wire. More importantly, the cross-sectional geometry of such a cable does not at all lend itself to termination in such a plug.
U.S. Pat. Nos. 277,248, 2,211,584 and 3,287,490 deal with the application of a conductive coating on a single insulated wire. The thicknesses of such coatings, however, make the resultant products impractical for incorporation in a multi-conductor cable that can be terminated in a modular plug.
Other prior art United States patents which relate generally to cable constructions or coatings include U.S. Pat. Nos.: 1,976,804; 2,161,395; 2,287,947; 3,211,821; 3,594,228; 3,792,192; 4,079,156; 4,081,602; and 4,130,854. However, none of the structures described in these patents solve the problems noted above.